Researchers from Russia’s Skolkovo Institute of Science and Technology (Skoltech) and Lomonosov Moscow State University have developed a sodium-ion battery cathode material that increases energy density.
The new material is a powder of sodium-vanadium phosphate fluoride (NaVPO4F) with a particular crystal structure that gave the researcher’s coin cell a practically achieved energy density of around 540Wh/kg.
The team developed the cathode active material by combining a NaVPO4F composition with a potassium, titanyl and phosphate group (KTiOPO4-type or KTP) framework via a low-temperature (190 °C) ion-exchange synthesis.
The team tested the material in a coin cell in combination with a sodium metal cathode and a NaPF6-based non-aqueous electrolyte solution.
This cathode active material enables a discharge capacity of 136mAh g−1 at 14.3 mA g−1 with an average cell discharge voltage of about 4.0V.
Furthermore, a specific discharge capacity of 123mAh g−1 at 5.7 A g−1 was also reported for the same cell configuration.
The research findings are reported in the journal Nature Communications.
A Skoltech spokesman told BEST: “Notably, this value was obtained in calculation of cathode material by multiplying its practical specific capacity at C/10 rate and an average discharge potential when tested versus sodium metal as anode. 10-15% energy density gain is referred to as a comparison with a state-of-the-art cathode material in sodium-ion batteries, which actually are also vanadium-based.
“Taking into account a chemical nature of elements, we can imagine potassium being replaced by sodium, titanium by vanadium and oxygen by fluorine; then we get a formula of NaVPO4F.
“So here we came to the conclusion that a material with this formula could be stabilised within a KTP-type structure because no chemical contradictions were made when composing the above-mentioned formula.
“According to our research, the structure of KTP-type NaVPO4F is built upon helical chains of corner-shared VO4F2 octahedra linked by PO4 tetrahedra through vertexes.”
Study co-author, assistant professor Stanislav Fedotov of Skoltech, said what made the new material and commercially available different was how those atoms were arranged and in what ratio they were contained in the compound.
He said: “Our material also compares well with the class of layered materials for cathodes. It provides roughly the same battery capacity and greater stability, which translates into longer life and higher cost-efficiency of the battery.
“Remarkably, even the theoretical predictions for the competing materials fall short of the practical performance of ours, and this is far from trivial, because the theoretical potential is never fully realised.”
“Higher energy storage capacity is just one of the advantages of this material. It also enables the cathode to operate at lower ambient temperatures, which is particularly relevant for Russia.”
